Building Blocks of Ancient Comets Discovered in Meteorite

A meteorite discovered on the La Paz Icefield in Antarctica held onto a deep secret — a sample of carbon left over from the formation of the oldest comets in the Solar System. Analysis of this carbon-rich (C-rich) sample from the earliest age of the Solar System revealed a makeup and structure similar to primitive comets found in the Kuiper Belt, orbiting in the distant reaches of our family of planets.

LaPaz Icefield 02342, like other meteorites, are remnants of asteroids which survived journeys through the Earth’s atmosphere, before being recovered on the ground. Asteroids, like comets, coalesced from the gas and dust which formed the Solar System. However, asteroids formed much closer to the Sun than comets, resulting in a lower concentration of water ice and carbon in their bodies.

“Asteroids are believed to grow by accretion, where dust particles grow into dust clumps that grow into bigger clumps that grow into rocks etc., eventually building up into much larger objects. This C-rich object probably didn’t land on a fully formed body, but rather probably accreted into a much smaller clump of material that eventually became part of the meteorite’s parent asteroid,” explained Dr. Larry Nittler of The Carnegie Institution for Science, in an exclusive interview with The Cosmic Companion.

It’s What’s Inside that Counts

The makeup of meteorites can vary greatly, depending on the origin of the asteroid from which they formed. By analyzing the meteorite, researchers are able to piece together the history of the sample. One class of these objects, carbonaceous chondrites, are believed to have formed beyond the orbit of Jupiter. LaPaz Icefield 02342 (LAP 02342) is a prime example of this class of meteorite.

“Objects of this size dominate the flux of extraterrestrial material on Earth, but really C-rich micrometeorites are extremely rare. They may be more abundant in space however, as they are likely more easily destroyed by heating coming through the atmosphere than other types. They may be even rarer as inclusions in meteorites; this is the first we have seen in a carbonaceous chondrite,” Dr. Nittler explains.

During the earliest age of our Solar System, gas pressure pushed a small amount of ancient cometary material closer to the Sun, where asteroids (as well as the rocky planets) reside. Just over three million years after the formation of our family of planets — long before the Earth fully formed — this piece of cometary material, roughly one-tenth of a millimeter (1/250th of an inch) in length, was captured by a growing asteroid.

Like most asteroids, this body was likely subject to collisions with other objects in the inner solar system, resulting in fragmentation. A small fragment of this asteroid then headed to Earth, where it landed in Antarctica. Protected within this body was the precious cargo of ancient cometary material, preserved like an insect in amber.

“When we look at primitive carbonaceous meteorites like this one, it’s not unusual to see rare, tiny blobs of carbon no more than a few microns in size but what caught our eye in this meteorite was how big the C-rich clast was — 100 microns is huge for this type of material. And because it’s so large it allowed us to use a whole array of techniques to analyze it, maximizing what we can learn about this type of material,” Dr. Jemma Davidson of Arizona State University told The Cosmic Companion.

Carbonaceous (C) chondrites are among the most complex of the all known forms of meteorites. Containing rich organic compounds and water-bearing minerals, these primitive objects are among the rarest of all meteorites. These bodies may have played an important role in bringing water to the Earth during the earliest days of our planet. The research team subjected the sample to a wide range of tests, attempting to glean every secret they could from the precious geological cargo tucked within its matrix.

“We were particularly interested in oxygen and carbon isotopes because they help us identify presolar grains, microscopic pieces of stardust formed by stars that predate the solar system, which are present in high abundance in the C-rich clast. We were also able to characterize the carbon isotopes of the C-rich material and used spectroscopy to determine the organic functional chemistry of the carbon,” Davidson describes.

A Cold Reception Here on Earth

Numerous meteorites have been found in Antarctica’s La Paz Icefield, providing astronomers, astrophysicists and geologists a wealth of data in their study of the ancient Solar System. The string of events which encased this sample before its fiery journey through the atmosphere of our planet was exceptionally fortunate for researchers.

The La Paz Icefield in Antarctica is a “hotbed” of places to find meteorites, like this one, LAP 02205. Image credit: NASA

“Because this sample of cometary building block material was swallowed by an asteroid and preserved inside this meteorite, it was protected from the ravages of entering Earth’s atmosphere. It gave us a peek at material that would not have survived to reach our planet’s surface on its own, helping us to understand the early solar system’s chemistry,” Nittler explained.

It was a long journey for this tiny piece of carbon-rich material, during a trip lasting billions of years. But, the knowledge gained by studying this rare sample will enrich the study of comets, our Solar System, and our own home planet.